1. Technical Field
The present disclosure relates to a relay device and a wireless control network management system using the same.
2. Related Art
A technique in which, for example, a process control system in industrial automation is formed as a wireless control network system using wireless communication has been heretofore proposed. This aims at eliminating a disadvantage of lowering of control accuracy in a background-art control system which was formed as a wireline network system, so that a field device such as a sensor for measuring temperature, flow rate, etc. could not be set in an optimal position in a plant because of a communication distance limit, a hardwiring limit, etc. On the other hand, a wireless control network management system for controlling operation of the wireless control network system to optimize operation of the whole plant has been proposed.
Incidentally, various kinds of field devices such as a differential pressure gage, a flowmeter, a thermometer, a surveillance camera, an actuator, a controller, etc. are used as examples of the field device.
There has been recently a trend to incorporate a wireless technique in various industrial fields inclusive of a process control field, and wireless communication standards have been discussed in conjunction with the trend. Specifically, WirelessHART defined in HART (Highway Addressable Remote Transducer) 7 and a process control wireless communication standard ISA100.11a approved by the ISA100 committee in the International Society of Automation (ISA) have been discussed. WirelessHART and ISA100.11a are protocols/standards for industrial wireless communication using frequency division communication. For example, the contents of rules concerned with the respective communications have been described in the following documents.
Document 1: Wireless systems for industrial automation: Process control and related applications
Document 2: Control with WirelessHART
(URL:http://www.hartcomm.org/protocol/training/resources/wiHART_resources/Control_with_WirelessHART.pdf)
WirelessHART and ISA100.11a are used in a wireless control network system in which a great deal of field devices are set while size reduction and power saving characteristic of IEEE (Institute of Electrical and Electronic Engineers) 802.15.4 is utilized.
On the other hand, a wireless communication system using “Wi-Fi (Wireless Fidelity) technique (hereinafter referred to as Wi-Fi wireless communication)” according to IEEE 802.11a/IEEE 802.11b has been heretofore proposed.
With respect to the Wi-Fi communication, a wireless communication system in which field devices such as a maintenance terminal for field worker's maintenance work, a surveillance camera requiring communication of a great deal of data (e.g. statistical data and various kinds of image data such as motion image data and still image data), etc. are connected has been proposed in a process control system.
A relay device for relaying wireless communication according to WirelessHART and ISA100.11a and wireless communication according to Wi-Fi communication in order to control operation of a wireless control network system by using both the communications to optimize operation of a plant as a whole, and a wireless control network system using the relay device have been heretofore discussed.
In other words, a relay device for achieving a mechanism of coexistence of a wireless system according to WirelessHART and ISA100.11a using frequency hopping and a Wi-Fi system, and a wireless control network system using the relay device have been discussed.
For example, U.S. Patent Application Pub. No. 2007/026884 describes a relay device and a wireless control network system using the relay device in a related-art.
A technique concerned with a mechanism of coexistence of a wireless system using frequency hopping and a Wi-Fi wireless communication system has been described in US2007/026884. The technique is characterized in that communication frames having guard intervals (blank times for preventing interference) provided by dividing communication time zones of both wireless systems are used.
(Problem 1)
In the above-mentioned relay device and the wireless control network system in the related art, however, because the communication time zone in each communication system is divided, the time zone in which a wireless communication band can be used is limited in accordance with each communication system (each of the wireless system according to ISA100.11a etc. using frequency hopping and the Wi-Fi wireless communication system). There is a problem that throughput in each wireless system is lowered.
Moreover, in the above-mentioned relay device and the wireless control network system in the related art, because the communication time zone in each communication system is divided, the communication time allocated to each wireless system (each of the wireless system according to ISA100.11a etc. using frequency hopping and the Wi-Fi wireless communication system) is reduced compared with the case where only one wireless system is operated singly. Accordingly, there is a problem that the number of nodes accommodated per unit time is reduced in proportion to reduction in the communication time allocated to each wireless system.
Specifically, as a typical application in industrial wiring communication, there is an application which collects data of a large number of wireless devices periodically through an access point (hereinafter referred to as AP) which is an example of the relay device. The bottleneck of the number of accommodated nodes in this case is the AP. Particularly the number of communications per period allowed to be performed by the AP is directly connected with the number of nodes accommodated in the network.
For this reason, in accordance with the related art, the communication time zone in each communication system is divided. Accordingly, the communication time allocated to each wireless system is reduced compared with the case where only one wireless system is operated singly. There is a problem that the number of accommodated nodes per unit time is reduced in proportion to reduction in the communication time allocated to each wireless system.
(Problem 2)
On the other hand, it is conceivable that the mechanism of coexistence of the wireless system according to WirelessHART and ISA100.11a using frequency hopping and the Wi-Fi system is achieved in such a manner that channels allowed to be used in both the wireless systems are allocated stationarily.
However, if channels which can be used in the two wireless systems are allocated stationarily for coexistence though frequency division communication of ISA100.11a aims at reducing a risk of wireless communication fault such as noise interference by distributive hopping of channels to be used, mixing of Wi-Fi communication reduces channels which can be used for frequency hopping as will be clear from FIG. 7. Accordingly, there is a problem that not only the benefit of channel distribution according to ISA100.11a decreases but also the risk of deterioration of wireless communication quality due to noise interference increases.
Moreover, if a plurality of Wi-Fi networks coexist when usable channels are allocated stationarily to wireless systems according to respective wireless communication standards to attain coexistence, the number of channels allowed to be used in ISA100.11a is limited to a very small number. Accordingly, there is a problem that the advantage of the communication mechanism of ISA100.11a intended for high reliability in field environment is spoiled greatly.
An example of the related art in the case where usable channels are allocated stationarily to two wireless systems to make the two wireless systems coexistent and the problem inherent in the example of the background art will be described specifically with reference to the drawings.
FIG. 5 is a diagram for explaining the problem in a relay device and a wireless control network management system using the relay device in the related art. In FIG. 5, the related-art wireless control network management system includes a management server 1 which aggregates information of a wireless network from wireless nodes 51-52 and collects and stores data from the wireless nodes 51-52 to thereby control the whole of a wireless system, for example, according to WirelessHART and ISA100.11a using frequency hopping; AP's 2-4 as examples of relay devices which communicate with respective wireless nodes 51-52 and 61-63 and relay wireless communication; wireless nodes 51-52 (represented by ISA nodes in FIG. 5) which are field devices, for example, having sensor functions for measuring physical quantities such as temperature, flow rate, etc. or actuator functions for controlling control valves and which have wireless communication functions for wirelessly transmitting various kinds of measured data by wireless communication according to WirelessHART and ISA100.11a using frequency hopping; and wireless nodes 61-63 (represented by Wi-Fi nodes in FIG. 5) which are field devices such as a maintenance terminal for field worker's maintenance work, a surveillance camera requiring communication with a great deal of data (e.g. statistical data and various kinds of image data such as motion image data and still image data) and which have wireless communication functions for wirelessly transmitting various kinds of measured data by wireless communication according to Wi-Fi.
Here, the wireless nodes 51 and 52 perform wireless communication according to ISA100.11a and use IEEE802.15.4 with a 2.4 GHz band as a communication frequency band.
The wireless nodes 61-63 perform wireless communication according to Wi-Fi wireless communication. When IEEE802.11b/g is used as a communication frequency, the same 2.4 GHz band as that of ISA100.11a is used as a use frequency band. That is, the frequency band for wireless communication according to Wi-Fi and the frequency band for wireless communication according to ISA100.11a/WirelessHART compete with each other.
For this reason, the frequency band for wireless communication of the wireless nodes 51-52 and the frequency band for wireless communication of the wireless nodes 61-63 compete with each other because the same 2.4 GHz band is used.
FIGS. 6A and 6B are explanatory views showing a state of use of frequency bands when a wireless system using frequency hopping in FIG. 5 and a Wi-Fi system coexist. FIG. 6A is a view for explaining the frequency band of ISA100.11a wireless communication. FIG. 6B is a view for explaining the frequency band of Wi-Fi wireless communication.
FIGS. 6A and 6B show frequency bands used in respective wireless nodes. For example, the wireless nodes 51 and 52 perform wireless communication according to ISA100.11a ad use IEEE802.15.4 with a 2.4 GHz band as a communication frequency band. As shown in FIG. 6A, the communication channel is divided into sixteen communication channels (ID: 11 to 26).
The wireless nodes 61-63 perform wireless communication according to Wi-Fi. Particularly when IEEE802.11b/g is used, the wireless nodes 61-63 use the same 2.4 GHz band as that of ISA100.11a as a use frequency band. As shown in FIG. 6B, the communication channel is divided into three communication channels (ID: 1, 7 and 13).
Specifically, communication channels 11-14, 16-20 and 21-25 in the wireless nodes 51 and 52 compete with communication channels 1, 7 and 13 in the wireless nodes 61-63.
Meanwhile, wireless communication according to ISA100.11a/WirelessHART in the wireless nodes 51-52 is characterized in that communication is performed with time division multiple access (TDMA) and frequency division multiple access (FDMA) being combined with each other. The wireless communication is performed while a superframe which will be described later is used for defining frequencies and timings of communication between wireless nodes.
The management server 1 divides the time zone and frequency band of wireless communication performed by the wireless nodes 51-52, defines timings of communication between the wireless nodes and a frequency/frequency band in accordance with each communication timing, generates a superframe and distributes the superframe to the wireless nodes 51-52.
Incidentally, the term “superframe” means a communication template in which the timing of communication between wireless nodes and the frequency/frequency band for each communication timing are defined by dividing the time zone and the frequency band of wireless communication performed by the wireless nodes.
FIG. 7 is an explanatory view showing a superframe used in wireless communication according to WirelessHART and ISA100.11a using frequency hopping. For example, in FIG. 7, time zones and frequency bands (communication channels) of wireless communication according to ISA100.11a in the wireless nodes 51-52 are defined in the superframe. That is, frequency bands (communication channels) are allocated to divided time zones in such a manner that a communication channel Ch14 is allocated to a time zone t1-t2, a communication channel Ch12 is allocated to a time zone t2-t3, a communication channel Ch16 is allocated to a time zone t3-t4, . . . .
In this configuration, the management server generally has no mechanism of dynamically changing the frequency in wireless communication according to Wi-Fi. It is therefore necessary to allocate usable channels to the two wireless systems stationarily so that interference with the other wireless system can be avoided to avoid competition between frequency bands used in wireless communication according to Wi-Fi and wireless communication according to ISA100.11a/WirelessHART in a time zone to make channels allowed to be used in the two wireless systems coexistent.
For this reason, the management server 1 allocates usable channels to two wireless systems stationarily to avoid interference with the other wireless system and distributes communication setting information in which frequencies and timings of communication between wireless nodes are defined, to the respective wireless nodes 51-52 an 61-63.
Each wireless node performs communication based on a time schedule determined in a fixed communication channel set to avoid interference with the other wireless system based on communication setting information received from the management server 1.
As a result, when usable channels are allocated to two wireless systems stationarily in the aforementioned configuration, two or more wireless systems using different wireless communication standards can be made coexistent.
However, although frequency division communication of ISA100.11a aims at reducing a risk of wireless communication fault such as noise interference by distributive hopping of channels to be used, mixing of Wi-Fi communication reduces channels which can be used for frequency hopping as will be apparent from FIG. 7. Accordingly, there is a problem that the risk of deterioration of wireless communication quality due to noise interference increases.
Moreover, if a plurality of Wi-Fi networks coexist, the number of channels allowed to be used in ISA100.11a is limited to a very small number. Accordingly, there is a problem that the advantage of the communication mechanism of ISA100.11a intended for high reliability in field environment is spoiled greatly.